This invention relates to a method of treating metal surfaces to enhance corrosion resistant and paint bonding characteristics and more particularly, relates to trivalent chromium coatings for aluminum and aluminum alloys and, sealers for anodized aluminum substrates.
It is generally known to treat the surfaces of metals, such as zinc, cadmium, or aluminum with aqueous chromate (hexavalent chromium) solutions which contain chemicals that dissolve the surface of the metal and form insoluble films known as "chromate conversion coatings." These chromium coatings, are corrosion resistant and protect the metal from various elements which cause corrosion. In addition, it is known that chromate conversion coatings generally have good paint bonding characteristics and, therefore, provide an excellent base for paint or other finishes.
Although the aforementioned coatings enhance corrosion resistant and paint bonding properties, the coatings have a serious drawback, i.e., the toxic nature of the hexavalent chromium constituent. This is a serious problem from two viewpoints, one being the handling of the solution by operators and the other, the disposal of the used solution. The disposal problem, however, can be mitigated by reducing the hexavalent chromium to the comparatively innocuous trivalent form before disposal. This method is expensive and therefore can be a major cost factor in the overall metal treating process. Therefore, it is highly desirable to have coatings which are substantially free of hexavalent chromium, but at the same time capable of imparting corrosion resistant and paint bonding properties which are comparable to those imparted by conventional chromium coatings.
Of particular interest is the use of chromate conversion coatings on aircraft aluminum alloys due to the excellent corrosion resistance and the ability to serve as an effective base for paint. The baths used to develop these coatings contain chromates, i.e., hexavalent chromium, and it is the residual chromates in the coating that is largely responsible for the high degree of corrosion inhibition. However, these same chromates are highly toxic and their presence in waste water effluents is severely restricted. It would therefore, be desirable to provide a coating for aluminum and its alloys and for sealing of anodized aluminum utilizing relatively non-toxic chemicals that could serve as an alternative to the toxic hexavalent chromate coatings.
In the prior art, trivalent chromium baths (U.S. Pat. No. 4,171,231) have been used to produce coatings on zinc and zinc plate to provide a decorative "clear to light blue finish" which are characterized as having superior corrosion resistance. These baths contain "trivalent" chromium as substantially the only chromium ion, with a fluoride ion, an acid other than nitric acid and an oxidizing agent. The operating range of the baths is at a pH between about 2 to 4 and preferably between 1 to 3. The baths are used to achieve a single-dip chromate finish on all types of zinc plate. The implication is that the presence of the oxidizer, in situ, produces hexavalent chromium on the zinc surface without any oxidation or conversion of the trivalent chromium in the bath to the hexavalent form. Patentee discloses that without the oxidizing agent in the bath, corrosion resistance was poor, i.e., extensive corrosion after 24 hours with a 5% salt spray exposure, whereas with the oxidizing agent in the bath there was 0-10% of white corrosion and some panels were free of white salt after 50 hours of salt spray exposure.
This invention, in comparison, utilizes trivalent chromium as the only chromium ion in the bath with a fluoride ion, preferably from a complex compound such as a fluosilicate at a specific pH range. It was found that the addition of an oxidizing agent such as peroxide to the bath, in situ, slowly oxidized the trivalent chromium to the toxic hexavalent form. This conversion to the hexavalent form is contrary to the method used by this invention; namely, utilization of a bath composition completely free of hexavalent chromium.